Watch this video where I walk you through step by step as I create a record-triggered before-save flow and use workarounds to utilize ISCHANGED/ISNEW functionality. This video was recorded using a Winter 21 preview instance. So the short answer to the title question is that we still cannot use ISCHANGED/ISNEW, but the long answer is that we can use workarounds to get this functionality in a before-save flow. And all this became even more efficient in Winter 21.
Please note that there is an error in logic I discovered after I recorded this video. I needed to add to the FromAnotherStage decision outcome condition ‘AND PrevStage Does Not Equal to Closed Won’ to allow for NotChanged to work correctly. Since I don’t do anything under NotChanged condition it did not affect the outcome here.
Please see the correct version here:
Please read this blog post for an example on Criteria Enhancement and Advanced Decision Elements in Winter 21 Release.